Emediately put out the mouthpiece, raise their feet a little (put a towel under it), do a chin-lift and a hyper-extension of the neck (put your hand on the forehead and press back the head gently) this way he can't choke on his tongue, and check for other vital functions (hear and feel if he still breaths), if he still breaths, he should slowly come back to conciousness within short time (+- 15 sec), if so keep him calm and let him lay there in that position for a while untill he is fully recovered to get up, if he does not regain conciousness after those 15 secs, you will have to inform an ambulance, keep paying attention that the victim is not able to chocke in his own tongue by holding the hyper-extension or if you have to leave the victim alone for any reason, you put him on his side, so he can't choke in his own tongue, or choke because he s throwing up.
A cold compress on the forehead is a good extra measure as well, to get him regain conciousness faster and to ease the headache.